Ion-permeable battery separator and method of making same



? 1965 s. M. PARKER ETAL 3,216,863

ION-PERMEABLE BATTERY SEPARATOR AND METHOD OF MAKING SAME Filed Dec. 28, 1961 VI IIIIIIIIII IIIIIIIIII II IIIIII E&$\\\\\\\\\\\\ Fig.3

United States Patent C This invention is concerned with plate separators for storage batteries of the lead-acid type and especially with a plate separator which is non-porous but ion permeable as contrasted with the porous or microporous conventional separator presently used.

The primary object of a battery separator is to prevent metallic conduction between the plates of opposite polarity while freely permitting electrolytic conduction. In a conventional lead-acid storage battery, the lead and lead compounds may grow from the plate outwardly forming structures which are known as trees. If the tree completely penetrates the pores of a separator, an internal short circuit develops and the battery fails. In addition to tree growth, there is also the problem of scaling of paste particles from the positive plate. The scaled particles may fall to the bottom the container where they do no harm. However, they may also lodge themselves in the pores of a conventional separator where such particles build up and make a metallic connection between the plates with consequent short circuiting. On the other hand, if the separator is non-porous and permeable only to ions in the electrolyte, neither trees nor shed particles can penetrate the membrane and internal short circuiting of the battery cannot take place.

Despite many previous attempts to produce ion permeable membranes for use in lead-acid storage batteries, so far as we are aware, such membranes have not been successful. Most of the compositions have oxidized badly when in contact with the positive battery plate and many have softened or swollen in the battery acid. Also, a serious diificulty has been the fact that if the material is sufiiciently stable under the oxidizing and acid conditions which exist in the cell, its electrical resistance (considered as a series resistance) is too high. The result is that the discharge rate of such a battery is much lower than is commercially permissible.

The objects of the present invention are to produce a non-porous ion-permeable battery separator which is low in resistance; to produce a separator which has high corrosion resistance in the environment of the cell; to produce a separator which is an effective barrier against scaling and treeing; to produce a separator which is wettable by the battery acid, is dimensionally stable, capable of maintaining the plate spacing in the cell, and which remains unaffected by its environment so that a long life for a battery may reasonably be expected.

These and other objects will become apparent from the specification and the drawing in which:

FIG. 1 is a projected view illustrating the component members of the separator in preassembled relationship.

FIG. 2 is a perspective view showing the assembled separator with the members facing the negative and positive plates partly rolled away to show the construction more clearly.

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FIG. 3 is a cross section of the assembled separator taken on the line 3-3 of FIG. 2.

Referring to the drawing, the separator 10 is essentially a three-part structure comprising two perforated sheets 11 and 12 of an inert substance with the imperforate ion permeable membrane 13 interposed between the sheets. Perforated sheet 11 which faces the negative plate) usually has a plane surface and is perforated except at the perimetrical margin 21 in an allover pattern of small holes 22 which removes approximately 50 percent of the area of the sheet.

The sheet 12 which faces the positive plate is similarly perforated with holes 23 except at the perimetrical margin 24 and for imperforate ribs 14, the latter of which extend in parallel lines from the top to the bottom of the separator in the manner of a conventional lead-acid separator. The height of the ribs is variable and is in accord with the specifications of the battery manufacturer. If the sheet is made by an extrusion process, the ribs are formed at the time that the sheet is extruded. They may also be rolled into the sheet or may be separately applied and adhered to the sheet either by heat fusion or by an adhesive.

The ion permeable membrane 13 is essentially the reaction product of a polyvinyl chloride and a substance the important ingredient of which is polyimidazoline. The polyimidazoline endows the polymer with thermoset properties. This reaction product has been described in an application for United States patent, by Elizabeth C. Dearborn and Philip K. lsaacs entitled Reaction Product of Oleic Acid, Sebacic Acid and Triethylene Tetramine, Serial No. 78,878, filed December 28, 1960, now Patent No. 3,050,528. The entire disclosure of this application is hereby incorporated by reference.

Various polyvinyl chlorides are suitable. Commercial resins of somewhat low molecular weight and fine particle size are preferred since the film which results from the polyimidazoline-polyvinyl chloride reaction appears to be stronger, and crazing and cracking of the film in the film casting process occurs less frequently than when hard high molecular weight resins are used.

The exact structure of the polyimidazoline which has given successful results is as yet unknown. The average composition as determined by infrared absorption analysis shows that approximately 80 percent of the carboxyl groups originally associated with the carboxylic acids have been converted to imidazoline rings. The remaining 20 percent of the carboxyl groups exist in the reaction product as amide groups. The average molecular weight of the reaction product lies in a range between about 3000 to 4500. The idealized polyimidazoline structure has been postulated as H2O ii J IL n, H 0

but the molecular weight of the idealized material is about 2000. In what follows, this reaction product will, for brevity, be called polyimidazoline.

The material, which forms the ion-permeable membrane 13, is made up into a spreadable compound suitable for film casting procedures by mixing polyvinyl chloride, the polyimidazoline, a phosphate ester type plasticizer, such as n-butyl, Z-ethylhexyl, phenyl, cresyl, tri(p-tertbutylphenyl), and such mixed phosphate esters as Z-ethylhexyl diphenyl, cresyl diphenyl and xenyl diphenyl phosphate, a very small proportion of an acid acceptor, such as zinc oxide, and a fugitive solvent, such as toluene. The ratio of polyvinyl chloride to polyimidazoline in CH2 CH2 these mixtures may vary between 2:1 to 5: 1. Preference is given to the lower ratio since it has been found that these ratios produce a film having lower electrical resistance. The proportion of the phosphate ester type plasticizer to the wet weight of the compound may range from to 20 percent, and desirably is about 14 percent. The proportion of the solvent, toluene, may be varied to give good spreading characteristics to the compound. \Vorkable proportions usually will be found in the range of 25 to 35 percent of the wet weight of the compound. The proportion of acid acceptor, zinc oxide, may range from 0.5 to 1 percent.

Example I illustrates the procedure by which the polyimidazoline is made:

Example I 49.0 lbs. (.17 mole) of oleic acid were charged to a glass vacuum vessel and then 87.6 lbs. (.43 mole) of sebacic acid were added thereto. The acids were heated to about 70 C. and then 76 lbs. (.52 mole) of triethylene tetramine were added. Due to the exothermic nature of the reaction between the amine and acids, the temperature of the mixture rose to about 100 C. 0.213 lb. of powdered sodium tripolyphosphate was then added as a metal chelating agent. Vigorous agitation and a nitrogen atmosphere were maintained throughout. The mixture was then. heated at 1 atmosphere to about 150 C. and the temperature and pressure were carefully controlled thereafter as follows:

After a temperature of about 220 C. and a pressure of about 15 mm. Hg have been reached according to the foregoing schedule, the pressure and temperature were held at these values and the reaction was continued for about one hour. During this period, water of condensation was continuously removed to avoid hydrolysis of the polyimidazoline. The amount of water removed over the entire reaction period corresponded with about 80 percent conversion of the carboxyl groups to polyimidazoline groups, leaving a balance of about percent of the carboxyl groups in the form of amides. Finally, the product was cooled under nitrogen to about 130 C.

Preparation of the material which is used to form the imperforate ion-permeable membrane 13 using the polyimidazoline of Example I is described in Example II.

Example 11 One part of polyimidazoline was weighed into a container, which was capped to prevent entrance of oxygen, and heated to between 50 and 55 C. 1.1 parts of 2-ethylhexyl diphenyl phosphate (Santicizer 141) was added to the warm reaction mixture and stirred until the ingredients were thoroughly blended together. The blended product was then strained through a fine mesh screen. 1.8 parts of toluol (Solvesso) were then slowly added to the strained mixture and stirred for approximately five minutes. 0.5 part of zinc oxide (Kadox and one patr of Z-ethylhexyl diphenyl phosphate were thoroughly worked together by a triple passage through a homogenizer. The .mixture of zinc oxide and diphenyl phosphate was then slowly added to the toluol-diluted material with stirring. Stirring was continued for ten minutes. The speed of mixing was then raised so that a vortex was formed in the liquid, and the temperature was allowed to drop to 38 C. At that time, 2% parts of polyvinyl chloride (Opalon 410) were added to the mixture. The polyvinyl chloride was added in small quantities so that lumping during mixing was avoided. As the mixture thickened, the speed of stirring Was increased sufiiciently to maintain the vortex at all times. Stirring was continued until the polyvinyl chloride was thoroughly incorporated (about 20 minutes). The mixture was then strained. In this example, all parts are by weight.

Ion-permeable membranes are cast from the mixture of Example II on a conventional plate film casting machine. Preferably the casting bed is a sheet of glass. The thickness of the liquid coat should be approximately twice that required for a dried film.

After the film has been spread on the glass plate, it is thoroughly dried in air (a mild heat is permissible). The film is cured on the plate in a forced draft oven at 177 C. for six minutes and then allowed to cool to room temperature. When room temperature is reached, the cured film is soaked with water to loosen it from its support, and subsequently dried. Films of 0.001 to 0.005 inch finished thickness have proved practical, e.g. multiple cast films of 0.025 to 0.003 inch thickness perform excellently.

The dried film is cut to appropriate size, laid on the perforated support member 11, and then covered with the perforated and ribbed support member 12. The perforations 22 and 23 occupy from 40 to 60 percent, e.g. 50 percent of the area of each sheet. The entire assembly may be held together with an acid-resistant solvent rubber cement 15 applied to the margins 21 and 24 of members 11 and 12, or the edges of the assembly may be fused together by heat-sealing. Heat-sealing is accomplished by pressing the assembly under a heated die which heats and compresses the marginal area.

Preferably the film should be built up in two or three steps. If a 0.003 inch sheet is to be made, it is recommended that a liquid sheet of 0.002 inch be first spread on the plate and this film allowed to dry thoroughly. Subsequent to drying, a second spread of 0.002 inch thickness should be made and, after drying, a third liquid spread of 0.002 inch follows. Despite the intermediate dryings, the film, after it is cured, shows no lamination and is a unitary product. Its dry thickness is 0.003 inch.

Multiple film casting has been found to produce superior ion permeable membranes which are completely free of pin holes. Pin holing sometimes occurs when the film casting takes place in a single step.

The following Example III gives a typical composition for preparing the ion-permeable membrane 13, showing its percent composition on a wet and a dry basis:

Example Ill The polyimidazoline is a strong organic base and is highly reactive with polyvinyl chloride. The polyimidazoline is unique in its ability to crosslink the polyvinyl chloride when the composition is heated at temperatures ranging between about C. and 240 C.

The essential quality of the suport elements 11 and 12 is inertness in the environment of a lead-acid cell, but the material also should lend itself to easy perforation without shattering, and be strong and tough to permit automatic interleaving, and permit rib rolling. A number of substances are suitable, but cost is a determining factor. Two substances which meet chemical and mechanical requirements and are low in cost are perforated polyvinyl chloride sheet and a sheet made from a high density polyethylene containing a small proportion of, e.g., percent butyl rubber. (Butyl rubber is a copolymer of isobutylene with a small amount, about 2 percent, of diolefin, such as isoprene.) Sheet thicknesses of 0.010- 0.020 are suitable. As an example, a complete separator may have a finished thickness (including rib thickness) of 105 mils and interpose a series resistance in a lead-acid cell of 62 milliohms per square inch. Perforations may range from the smallest practical punching diameters to inch. A inch perforations are preferred.

In use, the separators have proved to be rugged and lend themselves well to automatic assembly. The cells produce acceptable discharge rates and, especially when the ion-permeable membrane is made by the multiple-coat casting process, eliminate treeing. Corrosion tests of the membrane have shown that it does not weaken or develop pin holes. Present separators, including the microporous types, show considerable corrosion in a comparable test.

We claim:

1. A plate separator for lead-acid storage batteries comprising a non-porous, ion-permeable membrane supported between two perforated sheets of a substance inert in battery acid, said membrane being the dried and cured reaction product of a mixture of from 2 to 5 parts of polyvinyl chloride, one part of polyimidazoline, a phosphate ester plasticizer, and a minor proportion of zinc oxide, said membrane being prepared by forming a film of the mixture, drying the film and heating the dried film at a temperature and for a time sufficient to cure the mixture, said polyimidazoline being derived by reacting oleic acid, sebacic acid, and triethylene tetramine in a molar ratio of 2:5 :6 respectively in a nitrogen atmosphere at a temperature of about 150 to 220 C. and a pressure of to 760 mm. Hg while continuously removing the water of reaction and continuing the reaction until the flow of water substantially ceases.

2. A plate separator for lead-acid storage batteries as defined in claim 1 in which the ion permeable membrane is supported between perforated sheets of an inert plastic substance having a thickness of from 0.0010 to 0.0020 inch.

3. A plate separator as defined in claim 1 wherein the perforations in the supporting sheets occupy from 40 to 6 p c t o t e a ea f e eet a d herei the d ameter of the perforations is not in excess of A of an inch.

4. A plate separator for lead-acid storage batteries as defined in claim 1 wherein the assembly of plastic supporting sheets and the ion permeable membrane is maintained by an acid-resistant cement applied to the margins of each unit of the assembly, said cement forming an edge seal of the assembly at the margin of the separator.

5. The method of forming a plate separator suitable for use in lead-acid storage batteries which comprises, forming a spreadable mixture of from 2 to 5 parts of polyvinyl chloride, one part of polyimidazoline, a phosphate-ester plasticizer, a minor proportion of zinc and oxide and toluene, said polyimidazoline being derived by reacting oleic acid, sebacic acid, and triethylene tetramine in a molar ratio of 2:5 :6 respectively in a nitrogen atmosphere at a temperature of about to 220 C. and a pressure of 15 to 760 mm. Hg while continuously removing the Water of reaction and continuing the reaction until the flow of water substantially ceases, spreading the mixture on a support to form a film, removing the toluene by drying the film, reacting the dried film by heating it to approximately 177 C. for a period of approximately six minutes, removing the film from the support, cutting the film to separator size, interposing the film between perforated sheets of a plastic substance inert in the battery acid, and edge sealing the supporting sheets and the ion permeable membrane to form a unitary plate separator structure.

6. The method of claim 5 wherein the liquid mixture is spread on a casting plate in a plurality of coats and wherein each separate coat is dried prior to subjecting the multiple coatings to the reaction temperature.

References Cited by the Examiner UNITED STATES PATENTS 1,786,328 12/30 Benner et al. 136145.5 2,772,322 11/56 Witt et al 136145.4 3,036,143 5/62 Fisher et a1 136145 3,050,527 8/62 Dearbom et a1. 260309.6 3,050,528 8/62 Dearborn et a1. 260-3096 WINSTON A. DOUGLAS, Primary Examiner. JOHN MACK, Exqminer. 

1. A PLATE SEPARATOR FOR LEAD-ACID STORAGE BATTERIES COMPRISING A NON-POROUS, ION-PERMEABLE MEMBRANE SUPPORTED BETWEEN TWO PERFORATED SHEETS OF A SUBSTANCE INERT IN BATTERY ACID, SAID MEMBRANE BEING THE DRIED AND CURED REACTION PRODUCT OF A MIXTURE OF FROM 2 TO 5 PARTS OF POLYVINYL CHLORIDE, ONE PART OF POLYIMIDAZOLINE, A PHOSPHATE ESTER PLASTICIZER, AND A MONOR PROPORTON OF ZINC OXIDE, SAID MEMBRANE BEING PREPARED BY FORMING A FILM OF THE MIXTURE, DRYING THE FILM AND HEATING THE DRIED FILM AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO CURE THE MIXTURE, SAID POLYIMIDAZOLINE BEING DERIVED BY REACTING OLEIC ACID, SEBACIC ACID, AND TRIETHYLENE TETRAMINE IN A MOLAR RATIO OF 2:5:6 RESPECTIVELY IN A NITROGEN ATMOSPHERE AT A TEMPERATURE OF ABOUT 150 TO 220*C. AND A PRESSURE OF 15 TO 760 MM. HG WHILE CONTINUOUSLY REMOVING THE WATER OF REACTION AND CONTINUING THE REACTION UNTIL THE FLOW OF WATER SUBSTANTIALLY CEASES. 